The present invention relates to a brake pressure regulation method and system for a trailer vehicle which is connected to a tractor vehicle having an electronically controlled pressure-medium brake. The term "trailer vehicle" should here be taken to refer both to drawbar trailers and to semitrailers.
In vehicle combinations with conventional air brake systems, prescribed retardation bands are used in an attempt to achieve uniform participation of the tractor and the trailer vehicle in the braking work, i.e. the braking action both of the tractor and of the trailer vehicle must lie within certain band limits in relation to the pressure at the trailer connection used as a reference. These limits are relatively wide taking into account various limitations of conventional brake systems. In practice, this then leads in some cases to brake force distributions exhibiting a considerable error, especially in the range of the frequently occurring adaptive braking operations at a relatively low deceleration level. This results, inter alia, in extreme differences between the tractor and the trailer vehicle as regards the brake lining wear.
In electropneumatic brake systems, i.e. electrically/electronically controlled brake systems with wheel brakes actuated by compressed air, so-called coupling-force control systems are used in an attempt to improve the ratios. The function of these control systems is to detect "incorrect" brake pressure distributions and to influence the trailer control valve situated in the tractor in such a way that the forces between the tractor and the trailer vehicle are minimized.
German Offenlegungsschrift 3,901,270, for example, discloses an air brake arrangement for motor vehicles in which a sensor is arranged at a mechanical connection device for the vehicles in order to balance the braking action between the tractor and the trailer. The sensor measures the forces which occur at the connection device and controls a correction valve which is connected to the trailer supply line. Control is exercised via a control unit in which a control algorithm is installed to record and modify a pressure value and to emit signals which can be used to set a particular pressure in the trailer brake line.
German Auslegeschrift 2,164,325 also describes an apparatus for regulating the braking force at the wheels of motor vehicle trailers by influencing the pressure of the brake fluid for the trailer. This influence is exerted as a function of certain signals of a measuring element provided in the connection between the tractor vehicle and the trailer in order to measure the force occurring there. However, coupling-force sensors for rough conditions, as required by this apparatus or arrangement and by corresponding braking methods, are expensive and have to be continuously checked.
It is therefore an object of the present invention to provide a brake pressure regulation method for a trailer vehicle which is connected to a tractor vehicle having an electronically controlled pressure-medium brake which manages without sensing the forces between the tractor and the trailer vehicle. It is thus possible to dispense with corresponding coupling-force sensors and to have a method universally applicable, especially in conjunction with modern, electronically controlled power brake systems with ABS in tractor vehicles.
These objects have been achieved in a method for brake pressure regulation on a trailer vehicle including the steps of
(a) obtaining wheel speed signals from non-driven wheels of the tractor vehicle and of the trailer vehicle;
(b) transmitting at least one signal representative of at least the wheel speeds of one axle of the trailer vehicle to the tractor vehicle;
(c) cyclically processing, together with wheel speed signals from the tractor vehicle, the at least one signal, representative of at least the wheel speeds of one axle of the trailer vehicle, in the electronic control unit, including
(d) continuously forming and updating a sliding average (ds.sub.U) of an instantaneous relative difference (ds.sub.A) between average wheel speeds of non-driven wheels of the tractor vehicle and of the trailer vehicle in the unbraked condition from the beginning of a journey;
(e) in the case of a braking operation, calculating and making available the brake pressure for the trailer vehicle (P.sub.trail), starting from a fixed starting value (PHIA.sub.o) representative of the tractor vehicle for a brake pressure relation between the tractor vehicle and the trailer vehicle, as a product of a reference brake pressure of the tractor vehicle (P.sub.tractor) and a pressure relation coefficient (PHIA);
(f) after the beginning of the braking operation, calculating the actual retardation (Z.sub.actual) from the speed (V.sub.VA) of non-driven wheels of the tractor vehicle;
(g) as braking continues, making a check, by analyzing the trailer brake pressure (P.sub.trail), the instantaneous relative difference (ds.sub.A) between average wheel speeds of non-driven wheels of the tractor vehicle and of the trailer vehicle and the actual retardation (Z.sub.actual), to determine whether a steady-state braking phase is present;
(h) making a check to determine whether a difference (dds) between the instantaneous relative difference (ds.sub.A) between average wheel speeds of non-driven wheels of the tractor vehicle and of the trailer vehicle over said steady state braking phase and the unbraked updated sliding average thereof (ds.sub.U) has a larger absolute value than a threshold value EPSILON;
(i) if a steady-state braking phase is present and EPSILON is exceeded, then, calculating a change (dPHIA) in the brake pressure relation between the tractor vehicle and the trailer vehicle, this change being proportional to the current difference (dds) between said instantaneous relative difference (ds.sub.A) between average wheel speeds of non-driven wheels of the tractor vehicle and of the trailer vehicle and the unbraked updated sliding average thereof (ds.sub.U);
(j) adding the change (dPHIA) to the previous value (PHIA.sub.o, PHIA); and
(k) storing the resulting pressure relation coefficient (PHIA) adapted in the foregoing steps as the new pilot control value for distributing the brake pressure between the tractor vehicle and the trailer vehicle for the current journey.
Accordingly, the method of the present invention advantageously renders superfluous apparatus for detecting the coupling force between the tractor vehicle and the trailer vehicle since control is performed such that the mean utilization of adhesion at the axles of the trailer vehicle is as great as at those of the tractor vehicle. This aim is achieved by transmission of wheel speed signals from the trailer vehicle to the tractor vehicle since, in this way, the speeds of the wheels of all the axles are in the end adjusted to the same, if required normalized, mean value, and the brake pressure for the trailer vehicle is corrected or updated to give optimum distribution of the braking work between the tractor vehicle and the trailer vehicle.
Starting from a fixed initial value for the brake pressure distribution between the tractor and the trailer vehicle, an adaptive predetermination of the correct distribution of the braking force and, to this extent, the relation between the brake pressure level in the tractor vehicle and that in the trailer vehicle is taken as the basis for each current control intervention. This advantageously minimizes the necessity for current interventions in the range of customary retardations. For this purpose, the respectively expedient braking force distribution is determined and stored as a function of the retardation level demanded by the driver at the brake pedal even before the occurrence of large wheel speed differences between the axles of the tractor and the trailer vehicle, and is adapted (i.e. corrected) in the course of subsequent braking demands to the respective current requirements.
Generally speaking, the method according to the present invention makes it possible to use existing wheel speed transmitters of a tractor-trailer combination in order, on one hand, to provide possible balanced wear and, on the other hand, optimum utilization of the total braking capacity of the tractor vehicle and the trailer vehicle. Furthermore, the present invention has the advantage of minimizing the number of critical or highly stressed signal transmitters and their necessary connection paths to an electronic control unit in the tractor vehicle. To this extent, the method also brings about a considerable increase in the reliability of a corresponding brake system.